The present invention finds as its environment pressurized lines such as piping, tanks and chambers (referred to herein, generally, as "chambers"), on which are installed safety relief valves positioned at various locations along the chamber. A general understanding of the environment is seen in the specification of U.S. Pat. No. 4,349,885 (Thompson). Conventional safety relief valves, such as the type HB valves manufactured by Crosby Valve and Gauge Company, Wrentham, Massachusetts, and type 1543D valves manufactured by Dresser Valve Co., Alexandria, Louisiana, include, at least, a valve port, in communication with the interior of the chamber, for providing a pressure outlet, and a closure element which is nominally biased against the valve port to seal the port. Typically, the bias is provided by a spring loaded assembly acting on the closure element. The portion of the port against which the closure element rests is often referred to as the seat. The safety valve is designed so that when the internal pressure in the chamber is below a pre-determined threshold, defined as the "set pressure", the valve port is sealed. However, when the pressure within the line exceeds the set pressure, the line pressure is sufficient to overcome the spring bias so that the closure element lifts from the seat and the medium within the chamber may freely escape. In the prior art, safety valves are designed for specifically desired set pressures for various applications.
Understandably, it is desirable to be able to test the safety valve to determine the value of the set pressure (within an acceptable degree of accuracy) on a particular valve and, among other things, to determine whether or not the set pressure has changed from its original, rated value, as installed. Some popular systems known in the art for testing set pressure are described in the Thompson specification (patent No. 4,349,885) and include, without limitation, bench testing systems, which require removal of the valve from the chamber, and certain "on-line" systems, which rely upon the "audible pop" as an indicator that the set pressure has been reached. Because of the shut down of some operations required by bench testing systems, and because of the noise factor and possible harzardous results associated with the "audible pop" systems, Thompson developed a set pressure testing system which provided for remote measurement of the set pressure during on-line operation of the valve. The Thompson device, generally, combines monitoring of a position transducer for determining the relative position of the closure element and the seat with monitoring of a force transducer representing a selectively applied force used to oppose the spring loaded biasing force. More detailed explanation of the Thompson system is found in the above-stated patent No. 4,349,885.
The Thompson device appears to rely upon detected displacement of the closure element by a predetermined distance. While determining, from displacement detection, that the closure element has lifted from its seat is an indication that the set pressure has been reached, it is also a possible indication that the set pressure has been exceeded, and that "pop" has already occurred. This is not an acceptable test procedure in many applications.